This invention relates to vacuum pumps and more particularly to those pumps known as molecular drag pumps.
Molecular drag pumps operate on the general principle that, at low pressures, gas molecules striking a fast moving surface can be given a velocity component from the moving surface. As a result, the molecules tend to take up the same direction of motion as the surface against which they strike, thus urging the molecules through the pump leaving a relatively lower pressure in the vicinity of the pump inlet.
Vacuum pumps operating on the basis of this principle were proposed circa 1910 by Gaede. They generally comprised a cylinder adapted for rotation within a pump body and having a plurality of parallel slots, around its circumference. A stator element, sometimes referred to as a "comb", is supported within the body at one side of the pump and having parallel projections which fit closely within the slots, typically with a 0.1 mm clearance on all sides.
A pressure gradient is therefore established across the stator element with lower pressure on the upstream side and higher pressure on the downstream side. A pump inlet is positioned at the lower pressure side of the stator and an outlet at the higher pressure side and generally a separate pump, for example an oil pump, is connected to the outlet.
Generally the speeds of rotation of the cylinder are high, for example up to ten thousand revolutions/minute or more. In the case of relatively large machines of this type in particular, problems can arise due to the large mass of the rotor and hence of large inertia giving rise to a large amount of stored energy during rotation; this could lead to safety problems.
The invention is concerned with an improved pump design associated with the provision of a rotatable cylinder therein of lightweight construction.